Huge swathes of forest land in the Democratic Republic of Congo's Virunga National Park are being destroyed for valuable charcoal by criminals backing one of the region's most notorious rebel groups, a rights group said.

Congo's illegal charcoal trade - worth an estimated $35 million a year - is fuelling the widespread deforestation of Africa's oldest national park, and a range of crimes including murder, forced labour and sex slavery, the Enough Project said.

Charcoal traffickers are helping to finance the Democratic Forces for the Liberation of Rwanda (FDLR), a rebel group linked to Rwanda's 1994 genocide, according to a report by the Enough Project, a policy group fighting to prevent genocide and atrocities.

The rebel group, which consists of former soldiers and Hutu militiamen behind the genocide, has waged wars against other armed groups and the government and is believed to be at the heart of instability in the region, observers say.

Ethnic rivalries, foreign invasions and competition for land have stoked conflict among eastern Congo's dozens of rebel groups over the last two decades, costing millions of lives.

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"Peacebuilding in Congo will be a losing game without addressing the complex business networks operating in the east," said the Enough Project's senior policy analyst Holly Dranginis.

Covering some 7,770 square kilometres, Virunga is Africa's most biodiverse national park, a UNESCO world heritage site,and home to endangered mountain gorillas.

The charcoal from Virunga, called ndobo, is made by cutting down and burning trees in the park, and its trade is one of the FDLR's most lucrative businesses, the Enough Project said.

The rebel group coerces local people to produce ndobo, killing or enslaving those who resist, the group's report said.

Demand for the charcoal is concentrated in Congo, yet smugglers also transport it to Uganda and Rwanda, where old growth forests have nearly disappeared, according to the report.

While the state is responding to the FDLR's other illicit activities, such as mineral smuggling and elephant poaching, little has been done to tackle the charcoal trade, the report found.

Given that households across the region depend on charcoal as their main fuel source, law enforcement and military efforts to end its trade must be backed by alternative fuel initiatives to prevent a fuel shortage among millions of people, it said.

"Time is running out to address the charcoal trade, which has operated for years with few successful interventions," Dranginis said in a statement as the report was released.